This invention relates to the development and synthesis of divalent europium activated alkaline earth and/or lanthanum aluminate phosphor particles having a uniform particle size distribution (0.05-5 xcexcm) and a method of forming such particles. More specifically, this invention relates to a development and method of formation of such particles from respective oxides, nitrates and organic precursors which form small particles that improve the stability, longer life time and good color saturation as required for flat panel display (FPD) and lamp applications.
Divalent europium (Eu2+) activated barium magnesium aluminate (BAM) phosphor, is currently used in plasma display panels (PDP) as a blue emitting component. When compared with other phosphors used for red and green, BAM phosphor exhibits lower lifetime with the vacuum ultraviolet (VUV) flux. Efforts are being made to improve the existing BAM or to develop new phosphors to satisfying all requirements.
U.S. Pat. No. 3,294,699 discloses the invention of BAM as a blue phosphor. According to U.S. Pat. No. 4,110,660, the blend containing BaF2, LiF, Al(OH)3 and Eu2O3 was fired in a H2 atmosphere in the temperature range of 1400xc2x0 C.-1650xc2x0 C. for a period of 3 to 6 hours to obtain blue emitting divalent europium activated barium-lithium aluminate phosphor, used in xerography lamps. Koichi Takahashi et al. proposed BAM phosphor as a high radiation blue emitting phosphor under excitation by vacuum ultraviolet rays having the wavelength shorter than 200 nm in U.S. Pat. No. 4,161,457. The above said BAM was prepared by calcinating respective metal salts such as carbonates, nitrates, sulfates or halides at temperatures in the range 1200xc2x0 C.-1600xc2x0 C. in two different firing cycles. U.S. Pat. Nos. 5,989,454 and 6,187,225 are directed to blue phosphors.
Multi-phase structured Eu2+ activated La, Mg aluminate phosphor exhibited improved brightness over prior art single-phase compositions (U.S. Pat. No. 4,249,108). Starting materials (La2O3, MgO, Al(OH)3 and Eu2O3) were fired at 1500xc2x0 C.-1650xc2x0 C. for 1-5 hours in a reducing atmosphere (75 vol. % H2, 25 vol. % of. N2). The prior art also reveals that Ba (up to 25%) can be replaced by Sr (U.S. Pat. No. 4,590,405). Kijima et al. synthesized aluminate phosphor comprising (a) at least one element selected from the group consisting of Ba, Sr and Ca, (b) Eu, (c) Mg and/or Zn, (d) optionally Mn and (e) Al by firing the respective oxides and/or hydroxides in a reducing atmosphere at a temperature of from 1200xc2x0 C.-1700xc2x0 C. over a period of 2-40 hours (U.S. Pat. No. 5,611,959).
A method of producing BAM without any fluoride compound (flux) in the starting ingredients was described in U.S. Pat. No. 5,879,586. According to this invention, the particle diameter (1-20 xcexcm) and shape of the particles could be controlled by firing the samples in a reducing atmosphere at higher temperatures (1600xc2x0 C.-2000xc2x0 C.). European Patent 0 418 902 A2 teaches the role of alkaline earth and zinc in europium activated BAM in three component phosphor blends in controlling the fluorescent lamp performance.
According to Japanese Patent (8115673), BaMgAl10O17:Eu2+ has better time-wise durability when compared to traditional BaMgAl14O23:Eu2+. This particular phosphor was synthesized by calcinating a barium compound (such as BaO, Ba(OH)2, BaCO3, etc), an europium compound (such as Eu2O3, EuF3, etc.), a magnesium compound (such as MgO, Mg(OH)3, Mg(CO3)4.Mg(OH)2.5H2O, etc.), an aluminum compound (Al2O3, Al(OH)3, etc.) and a flux (fluorides of Ba, Al or Mg), at least once for 2-40 hours at 1200-1700xc2x0 C. in a reducing atmosphere in the presence of steam. For better stability (maintenance) of BAM in plasma display applications, Zachau (WO 99/34389) suggested manganese as a co-activator along with europium in BAM.
The main application of large area plasma displays will be High Definition Television (HDTV) and high information content presentation. HDTV and similar types of display devices should have phosphors with high resolution and high brightness for high performance. This can be achieved only with thin phosphor screens consisting of small phosphor particles in a close rib structure or closed cell structure. Screens with small particles exhibit higher packing density and also need reduced binder content. Stability is another concern in selecting a phosphor. The degradation of phosphor should not be more than 10% before 10,000 hours of operation. Three phosphors (red, green and blue) currently used in PDP""s do not exhibit same lifetime (stability) and particle morphology. Due to their physical nature, all of the three phosphors need different rheology of phosphor paste as well as different screening processes. In PDP applications these phosphors exhibit different life times. Blue phosphor degrades very fast when compared to other green and red phosphors. With the operation, the color point (color coordinates) of blue phosphor (BAM) shifts towards green. This result compromises the performance of the display. In consideration of these problems, we have dedicated our efforts to improve the existing phosphors or develop new phosphors.
Accordingly, it is an objective of the present invention to provide a method of preparation of divalent europium activated alkaline earth aluminate with or without lanthanum phosphor having the empirical formula:
(AE2-x-yLaxEuy)Al10O17 
Wherein: AE=Ba, Sr, Ca, or Mg, 0xe2x89xa6xxe2x89xa61 and 0.01xe2x89xa6yxe2x89xa60.1.
The present invention compares the synthesis of europium activated alkaline earth aluminate phosphor with or without lanthanum by two different processes: a conventional solid-state reaction process and a sol-gel process. Depending upon the required particle size distribution, the sol-gel process is superior for preparing small particles (0.05-5 microns) and the solid-state reaction is for normal size particles (5-20 microns).
The sols are dispersions of colloidal particles in a liquid. The gravitational forces on the particles are negligible. From a sol, a gel is formed with an interconnected, rigid network, having sub-micrometer pores and a polymeric chain whose average length is of the order of microns. The particle size of the finished product is a function of the initial concentration of the starting sols, gelation process, drying of gels, calcination temperature and rate of cooling.
Sol-gel process offers many advantages over conventional methods in the synthesis of fine powders and particularly phosphor materials. Since all of the starting materials are mixed at the molecular level in a solution, a high degree of homogeneity is achievable. Doping of impurities (activators/co-activators/sensitizers) through solutions is straightforward, easy and effective. The pores in properly dried gels are often extremely small and the components of a homogenous gel are intimately mixed. The surface area of powders produced from sol-gel is very high, leading to lower processing temperatures.
Phosphor materials are extremely sensitive to impurities; even in ppb levels, the low-temperature process through a sol-gel process minimizes the potential for cross contamination. Some of the unwanted impurities left in the materials from conventional methods (high temperature solid state reaction) may pose a threat to the performance of a phosphor. As the size of the phosphor particle decreases, the probability of electron and hole capture to the impurity increases and the e-h localization enhances the recombination rate via the impurity. The optimum impurity concentration (activator) level can be further increased with small particle size. The present invention is related to the growth of Eu2+doped alkaline earth aluminate phosphor with or without lanthanum by sol-gel methods.
More specifically, the present invention provides compositions and a process for forming a Eu2+ doped alkaline earth aluminate phosphor with or without lanthanum having the empirical formula:
(AE2-x-yLaxEuy)Al10O17 
Wherein: AE=Ba, Sr, Ca, or Mg, 0xe2x89xa6xxe2x89xa61 and 0.01xe2x89xa6yxe2x89xa60.1
(1) reacting a dilute solution comprising a source of an alkaline earth, a source of lanthanum, a source of europium and an organic precursor providing a source of aluminum, in an acid medium to form a dilute gel (sol-gel process);
(2) converting the dilute gel into a xerogel powder (room temperature drying) or converting the dilute gel into an aerogel powder (vacuum drying); or converting the dilute gel into a gel powder (spray drying); and,
(3) thermally decomposing the powders obtained from the above, at specified temperatures.
FIG. 1 shows X-ray diffraction pattern of Eu2+ activated barium magnesium aluminate phosphor prepared from xerogel.
FIG. 2 illustrates scanning electron micrographs of Eu2+ activated barium magnesium aluminate phosphors prepared from a) xerogel, b) aerogel, c) spray dried powder and d) oxides and nitrates (solid state process).
FIG. 3 shows emission spectra of Eu2+ activated barium magnesium aluminate phosphor of present invention prepared from aerogel excited at 147 nm or 173 nm. The emission was recorded at room temperature.